Various structures have been devised for magnet rotators for a motor. The rotators are broadly categorized into two groups: one of which is so-called surface permanent magnet (SPM) rotator in which a permanent magnet is arranged on the surface of a magnetic pole, as illustrated in FIGS. 3A to 3C and 3F; and the other of which is an interior permanent magnet (IPM) rotator in which a permanent magnet is arranged inside a rotator, as illustrated in FIGS. 3D and 3E. The former SPM rotator is configured such that the permanent magnet arranged on the surface of the rotator opposes a stator with an air gap therebetween, and has such an advantage that the SPM rotator may be designed and produced more easily than the latter IPM rotator. On the other hand, the latter IPM rotator is superior in constructional reliability and has an advantage in that a high reluctance toque may be easily obtained. An outer magnet rotator illustrated in FIG. 3F has often an SPM structure since a magnet is not liable to be scattered.
In a permanent magnet rotator illustrated in FIGS. 3A to 3F, a permanent magnet is generally bonded to a surface or an inside of a soft-magnetic yoke which is made of an insulating laminate of silicon steel sheets, cast or forged, with use of an adhesive.
The rotation of the magnet rotator incorporated in a motor generates a centrifugal force as the motor rotates, magnetic attraction and repulsive force between the magnet rotator and a stator. In addition, vibration or the like is also generated with the rotation of the motor. Insufficient bonding strength between the magnet and the soft-magnetic yoke detaches and breaks down the magnet. Since a centrifugal force is proportional to nearly the second power of a rotation speed, the higher the rotation speed is, the more serious the problem becomes. This pronouncedly appears in a case of when a segment magnet is used as illustrated in FIGS. 3A to 3F, particularly an inner SPM rotator in which a magnet is arranged on an outer surface of a rotator as shown in FIGS. 3A to 3C. Even when a ring magnet is used, in which a plurality of magnetic poles may be formed by a single magnet, the clearance of an adhesion layer is increased and a softer adhesive is often used in order to prevent the magnet from being broken down due to a difference in linear expansion coefficient between the magnet and the soft-magnetic yoke at the time when temperature of the rotator changes. A clearance of the adhesion layer results in increase in dispersion of adhesion strength, displacement of an adhesion position or the like. In general, the soft adhesive is inferior in thermal stability and adhesive force. As described above, the adhesion for the magnet rotator has many technical problems irrespective of a shape of the magnet.
In view of concern for the above adhesion strength, a structurally reinforcing protective ring 3 made of nonmagnetic stainless steel, a reinforced plastic fiber and the like is wound around an outer periphery of a magnet 101 to increase strength of the inner SPM rotator, as illustrated in FIG. 4. In such a case, however, an effective air gap is extended so that it becomes difficult that a magnetic flux from the magnet reaches the rotator, which leads to lowering an output of the motor. Moreover, the metallic protective ring made of stainless steel and the like generates an eddy current loss to decrease efficiency of the motor. While Patent Documents 1 and 2 disclose comparative examples in which a magnet and a soft-magnetic yoke are integrally formed, it is apparent that they do not have a sufficient bonding strength between the magnet and the soft-magnetic yoke, since it is assumed to use a structurally reinforcing frame or protective ring. It is apparent that a sufficient bonding strength cannot be obtained between the magnet and the soft-magnetic yoke, and the soft-magnetic yoke is held only with an inner pressure of the ring magnet in Patent Documents 3 and 4, from the disclosure of Patent Document 3 that a ring magnet rendered wedge-shaped is wedged into a yoke making use of a macroscopic external shape of the magnet to prevent the ring magnet from being detached from the soft-magnetic yoke, and the disclosure of Patent Document 4 that a magnet is limited to a ring shaped one and a production method thereof. Patent Document 5 discloses that pre-compacting and compacting steps are performed to mold a ring shaped magnet. A ring shaped magnet is, however, joined to a soft-magnetic yoke by an adhesive, which is insufficient in bonding strength and reliability.
Patent Document 6 discloses that a bonded magnet powder and a soft-magnetic powder are integrally compression molded without using an adhesive, so that a sufficient mechanical strength is obtained as a rotator. In particular, an IPM rotator is integrally molded while preventing a crack due to residual stress generated by a difference of spring back between the bonded magnet powder and those of soft-magnetic powder, within a range of the shape described in Patent Document 6. However, a noticeable crack often appears in the soft-magnetic yoke in a case where the rotator is out of the shape defined in Patent Document 6 or where a radial thickness of the soft-magnetic yoke part is smaller than the magnet part. The crack significantly decreases the mechanical strength of a compact and is not preferable for a rotator for a motor. As a demand for improving efficiency of a motor increases and for reducing its weight, a magnet rotator tends to be more complicated in structure and smaller in thickness. Thus, such a shape is demanded, which is difficult to be integral molding unless residual stress generated in the vicinity of a joint face and a difference of spring back of particles itself between the bonded magnet powder and the soft-magnetic powder are reduced.
Patent Document 1: JP-A-2001-95185
Patent Document 2: JP-A-2003-32931
Patent Document 3: JP-A-05-326232
Patent Document 4: JP-A-07-169633
Patent Document 5: JP-A-2001-052921
Patent Document 6: JP-A-2005-20991